A fourth missing member of the company, Gui Minhai, a Swedish national, was paraded on Chinese state television in January, where he said he had turned himself in for a fatal driving accident 11 years ago.

Gui had failed to return to Hong Kong from a holiday in Thailand in October.

In a letter to Hong Kong police, the Interpol Guangdong Liaison Office, part of the southern Chinese province’s public security department, said the three men being held “were suspected to be involved in a case relating to a person named Gui, and were involved in illegal activities on the mainland.”

“Criminal compulsory measures were imposed on them and they were under investigation,” said the letter, released by Hong Kong police late Thursday.

Enclosed was also a letter from the fifth missing bookseller, Lee Bo, Hong Kong police said.

Lee’s case has sparked the strongest backlash as he was the only one of the men to have disappeared while in Hong Kong.

Lee was last seen at a book warehouse in his home city in December.

Lawmakers and activists have accused mainland authorities of snatching Lee from the city, contravening Hong Kong’s laws which do not allow Chinese police to operate within the territory.

The new letter from Lee said Chinese authorities had told him Hong Kong police wanted to meet with him on the mainland, according to the Hong Kong police statement.

“He stated that he did not need to meet with police at the moment. He would contact police should he need to meet with police,” the statement said.

It added that Lee’s wife had confirmed the letter was in her husband’s handwriting.

Hong Kong police have asked the Guangdong authorities to “assist in following up the situations” of the three booksellers revealed to be under investigation, and to reiterate to Lee that Hong Kong police wanted to meet with him, the statement said.

Hong Kong was handed back to China by Britain in 1997 and its freedoms and way of life are protected under a 50-year agreement.